The receptor on T cells that binds antigen in the context of cell-surface Ia molecules is being studied in order to understand activation and regulation of the immune response. The approach has been to isolate the antigen receptor and characterize its multiple components. The biochemical events accompany antigen interaction with the receptor have been intensively studied in order to identify the mechanisms of signal transduction in T cells. The antigen receptor on a murine T cell hybridoma has been immunoprecipitated with a monoclonal antibody that uniquely binds its antigen recognition structure. We have shown that the clonally determined Alpha and Beta chains are non-covalently associated with five additional chains, (Delta, Epsilon, Gamma, Zeta, p21), two of which are homodimers (Zeta, p21). Thus the receptor complex consists of nine chains. To extend our studies, antibodies binding the Delta, Epsilon, and Zeta chains have been prepared. These reagents have been used to characterize the receptor on normal peripheral T cells and to analyze the biosynthesis and assembly of the components of this multichain structure.